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Water in the Atmosphere

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Essential Points

1. Water is part of the atmosphere

2. Partial pressure, vapor pressure and

humidity

3. Condensation and Clouds

4. Why it Rains

5. Hail, Snow and other forms of water

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Water in the Atmosphere

Water vapor makes up 0-4% of the

atmosphere

Most variable component

Residence time of a water molecule in the

atmosphere = 10 days

1. Water is part of the atmosphere

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Partial Pressure

The pressure that would be exerted by

each gas in the atmosphere if all the other

gases were removed

Proportional to the number of molecules

present

2. Partial pressure, vapor pressure and humidity

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Partial Pressure

Gas Molecular

Weight

Partial

Pressure

Nitrogen 28 78%

Oxygen 32 21%

Argon 40 0.9%

Water Vapor 18 0-4%

Carbon

Dioxide

44 350 ppm

2. Partial pressure, vapor pressure and humidity

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Vapor Pressure

2. Partial pressure, vapor pressure and humidity

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Vapor Pressure

The pressure of gas necessary to keep aliquid or solid from evaporating

The pressure of a substance in the

atmosphere in equilibrium with a solid orliquid

If partial pressure > vapor pressure,material condenses

If partial pressure < vapor pressure,material evaporates

2. Partial pressure, vapor pressure and humidity

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Vapor Pressure of Water

0 C 32 F 6 mb

10 C 50 F 12 mb

20 C 68 F 23 mb

30 C 86 F 42 mb

40 C 104 F 74 mb

50 C 122 F 123 mb

2. Partial pressure, vapor pressure and humidity

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Vapor Pressure and Humidity

The higher the vapor pressure, the moremoisture the air can hold

Vapor pressure increases with temperature

If partial pressure (water vapor actually inair) stays constant, then

Humidity decreases as it gets hotter

Humidity increases as it gets cooler Coolertemperatures can actually be less

pleasant

2. Partial pressure, vapor pressure and humidity

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Boiling Water

When the vapor pressure of water =

atmospheric pressure, water boils

Since air pressure drops with altitude, sodoes the boiling point of water

2. Partial pressure, vapor pressure and humidity

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Boiling Point of Water

0 km 100 C 212 F

5 km 83C 181 F

10 km 73 C 163 F15 km 55 C 131 F

20 km 40 C 104 F

25 km 26 C 79 F30 km 14 C 57 F

2. Partial pressure, vapor pressure and humidity

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Partial Pressure and Vapor

Pressure Partial pressure = Pressure water vapor

actually exerts

Vapor pressure = Pressure water vaporcould exertif the atmosphere weresaturated

Most of the time the partial pressure is lessthan the vapor pressure

2. Partial pressure, vapor pressure and humidity

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Adiabatic Expansion

Adiabatic = No heat transfer

Air warms or cools solely by expansion or

compression Why? Expansion does work, energy comes

from internal heat

Dry air: 10 C/1000m (5.5 F/1000 feet) Moist air: 6 C/1000m (3.3 F/1000 ft)

3. Condensation and Clouds

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Lapse Rate and Stability

If air cools faster with altitude than10C/km, it is unstable

If air cools 6-10 C/km with altitude, it is

stable for dry air but not moist air

If air cools less than 6 C/km with altitude, it

is stable

If air is warmer at high altitude than at thesurface, it is inverted and extremely stable

3. Condensation and Clouds

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Inverted?

If warm air rises, why is it an inversion to

have warm air above cooler air?

Because normally, heating at the surfacekeeps air at the surface warmer

Thats why the troposphere is tropo

Warm air above is more stable, but alsoless common

1. Water is part of the atmosphere

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Cloud Formation

Most cloud droplets require nuclei to form

Most nuclei are hygroscopic (attract water)

It is much easier to form water dropletsthan ice crystals

3. Condensation and Clouds

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Why Dont Clouds Fall Down?

Its a matter ofTerminal Velocity the faster

things fall, the more air resistance they

meet

Skydiver 50 m/sec

Mouse 5 - 10 m/sec

Cloud droplet 1 cm/sec

The tiniest air motions keep such small

particles suspended

3. Condensation and Clouds

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Orographic Effects

Orographic Lifting

Clouds

Precipitation

Rain Shadows

Katabatic Winds

Chinook or Foehn Winds

3. Condensation and Clouds

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Orographic Effects

3. Condensation and Clouds

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Rain Forest, Washington

3. Condensation and Clouds

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Rain Shadow, Nevada

3. Condensation and Clouds

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Why Does it Rain?

Cloud droplets are about 1/100 mm indiameter

It takes over a million cloud droplets to

make one raindrop

Coalescence of cloud droplets? Too Slow

4. Why it Rains

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The Bergeron Process

Molecules are less tightly bound in liquidsthan in solids

Therefore vapor pressure of water isgreater than that of ice

When ice and water droplets are present,water vapor can be saturated with respectto water droplets but supersaturatedwith

respect to ice Therefore water can evaporate from

droplets and freeze onto ice.

4. Why it Rains

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Freezing Rain

4. Why it Rains

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Hail

Often associated with violent storms

Typically 1 cm or less

Can be 10 cm capable of causing majordamage

Stones typically show concentric structure

Probably form by repeated trips throughsupercooled regions

5. Hail, Snow and other forms of water

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5. Hail, Snow and other forms of water

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5. Hail, Snow and other forms of water

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5. Hail, Snow and other forms of water

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1. Water is part of the atmosphere

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1. Water is part of the atmosphere

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1. Water is part of the atmosphere

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Record Hailstone, SD 2010

1. Water is part of the atmosphere

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Fog

Radiation: Surface cools at night by

radiation

Advection: Warm air carried over coolsurface

Upslope: Small-scale orographic clouds

Steam: Warm water evaporates into cold

air

5. Hail, Snow and other forms of water

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Advection Fog, Labrador

5. Hail, Snow and other forms of water

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Upslope Fog

1. Water is part of the atmosphere

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Steam Fog

1. Water is part of the atmosphere

F t

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Frost

5. Hail, Snow and other forms of water

G d F

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Ground Frost

5. Hail, Snow and other forms of water

h

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Ice Shove

5. Hail, Snow and other forms of water

C dl I

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Candle Ice

5. Hail, Snow and other forms of water

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Pancake Ice

5. Hail, Snow and other forms of water

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Essential Points

1. Water is part of the atmosphere

2. Partial pressure, vapor pressure and

humidity

3. Condensation and Clouds

4. Why it Rains

5. Hail, Snow and other forms of water